FGD Blowdown Treatment & ZLD Engineering in Power Stations

Executive Summary: Wet Limestone-Gypsum Flue Gas Desulfurization (WFGD) systems in thermal coal-fired power stations generate highly corrosive, complex wastewater streams enriched with chlorides, fluorides, heavy metals (Hg, As, Se, Cd, Pb), and dissolved scaling salts (CaSO_4, MgSO_4). Achieving Zero Liquid Discharge (ZLD) requires a integrated process architecture—combining chemical precipitation, high-pressure membrane pre-concentration, thermal evaporation via forced circulation Multi-Effect Evaporator (MEE) or Mechanical Vapor Recompression (MVR), and final solid recovery using Agitated Thin Film Dryers (ATFD) or crystallizers. This guide outlines the process thermodynamics, mechanical design codes (ASME VIII-1, TEMA Class R), metallurgical selection matrix, and sizing equations required for robust power plant ZLD engineering.

1. High-Level Process Overview & FGD Wastewater Dynamics

In coal-fired power generation plants, Wet Flue Gas Desulfurization (WFGD) scrubbers capture sulfur dioxide (SO_2) emissions by scrubbing boiler flue gas with an aqueous limestone (CaCO_3) slurry. The primary chemical reaction producing commercial-grade synthetic gypsum is represented as:

SO_2(g) + CaCO_3(s) + (1) / (2)O_2(g) + 2H_2O(l) \longrightarrow CaSO_4 ·  2H_2O(s)\downarrow + CO_2(g)\uparrow

As flue gas passes through the absorber tower, volatile halide species—primarily hydrogen chloride (HCl) and hydrogen fluoride (HF)—and trace metals volatilized during coal combustion dissolve directly into the recirculating slurry loop.

Flue Gas + Limestone Slurry 
            │
            ▼
 ┌──────────────────────┐
 │  WFGD Scrubber Loop  │ ──► Gypsum Cake (Dewatered)
 └──────────┬───────────┘
            │ Continuous Blowdown (12,000 - 20,000 mg/L Cl⁻)
            ▼
 ┌──────────────────────┐
 │ Heavy Metal Precip.  │ ──► Organosulfide / Lime / Ferric Dosing
 └──────────┬───────────┘
            │ Coagulated / Clarified Supernatant
            ▼
 ┌──────────────────────┐
 │ High-Pressure RO/EDR │ ──► Clean Permeate Water (Recycled)
 └──────────┬───────────┘
            │ Concentrated Brine (TDS > 80,000 mg/L)
            ▼
 ┌──────────────────────┐
 │ Forced-Circulation   │ ──► High-Purity Distillate
 │    MEE / MVR System  │
 └──────────┬───────────┘
            │ Concentrated Slurry (TDS > 300,000 mg/L)
            ▼
 ┌──────────────────────┐
 │ Crystallizer / ATFD  │ ──► Dry Mixed Salt Cake (< 5% Moisture)
 └──────────────────────┘

The Chloride Limit Challenge

Chlorides (Cl^-) do not precipitate as gypsum; they accumulate continuously in the liquid phase. Unchecked chloride buildup causes severe localized corrosion, pitting, and Stress Corrosion Cracking (SCC) in the scrubber metallurgy. Scrubber absorber loops are traditionally operated with a upper chloride threshold limit:

• SS316L Scrubbers: Cl^- \le 12,000 mg/L
• Duplex 2205 Scrubbers: Cl^- \le 20,000 mg/L
• Alloy 31 / Nickel-Alloy Scrubbers: Cl^- \le 40,000 mg/L

To maintain the chloride concentration below these material degradation thresholds, a continuous purge stream—termed FGD Wastewater Blowdown—must be bled from the scrubber system.

Environmental Mandates & ZLD Necessity

FGD blowdown contains severe toxic contaminants including bioaccumulative heavy metals (Hg, Se, As), fluorides, ammonia, and extremely high Total Dissolved Solids (TDS). Stringent regulations—such as the US EPA Steam Electric Power Generating Effluent Guidelines (ELGs) and Central Pollution Control Board (CPCB) ZLD mandates in Asia—prohibit direct discharge into water bodies. Consequently, power station utilities must install complete Zero Liquid Discharge (ZLD) process trains to achieve 100% water recovery and solid salt containment.

2. Chemical Pretreatment & Heavy Metal Precipitation

Raw FGD blowdown exhibits a low pH ($4.5 - 6.0$), high turbidity, supersaturated calcium sulfate (CaSO_4), and soluble heavy metal complexes. Direct thermal evaporation of raw blowdown causes catastrophic scaling and corrosion. Pretreatment requires multi-stage physical-chemical treatment.

Raw FGD Blowdown
       │
       ▼
 ┌───────────┐    Lime (Ca(OH)₂) [pH 8.5–9.2]
 │ Tank 1    │ ◄── Desaturation & Mg(OH)₂ Precipitation
 └─────┬─────┘
       │
       ▼
 ┌───────────┐    TMT-15 / Organosulfide (SDTC)
 │ Tank 2    │ ◄── Mercury (Hg²⁺), Cadmium (Cd²⁺), Lead (Pb²⁺) Removal
 └─────┬─────┘
       │
       ▼
 ┌───────────┐    Ferric Chloride (FeCl₃) / Coagulant
 │ Tank 3    │ ◄── Arsenate (AsV) & Selenite (SeIV) Co-precipitation
 └─────┬─────┘
       │
       ▼
 ┌───────────┐    Polyelectrolyte (Anionic PAM)
 │ Tank 4    │ ◄── Flocculation & Agglomeration
 └─────┬─────┘
       │
       ▼
 ┌───────────┐
 │ Clarifier │ ──► Metal Hydroxide / Gypsum Slurry to Filter Press
 └─────┬─────┘
       │ Clarified Overflow
       ▼
  To High-Pressure RO / MEE Evaporator

Reaction Chemistry & Dosing Regimens

1. Primary Desaturation & pH Neutralization

Hydrated lime (Ca(OH)_2) is dosed to elevate pH to $8.5 - 9.2$. This neutralizes residual acidity, precipitates soluble fluoride as insoluble calcium fluoride (CaF_2), and precipitates magnesium as hydroxide:

Mg^{2+} + 2OH^- \longrightarrow Mg(OH)_2\downarrow \quad (K_{sp} = 5.61  ×  10^{-12})

Ca^{2+} + 2F^- \longrightarrow CaF_2\downarrow \quad (K_{sp} = 3.45  ×  10^{-11})

2. Organosulfide Heavy Metal Precipitation

Soluble mercury (Hg^{2+}), cadmium (Cd^{2+}), lead (Pb^{2+}), and nickel (Ni^{2+}) form stable complexes that do not precipitate cleanly as hydroxides. Dosing trimercapto-s-triazine trisodium salt (TMT-15) or Sodium Dimethyldithiocarbamate (SDTC) precipitates extremely insoluble metal organosulfides across a wide pH band ($5 - 10$):

Hg^{2+} + SDTC^{2-} \longrightarrow Hg(SDTC)\downarrow \quad (K_{sp} < 10^{-50})

3Hg^{2+} + 2TMT^{3-} \longrightarrow Hg_3(TMT)_2\downarrow

3. Arsenic & Selenium Co-Preprecipitation

Arsenic (primarily arsenate HAsO_4^{2-}) and Selenium (selenite SeO_3^{2-}) are coprecipitated by adding Ferric Chloride (FeCl_3) at a dosage ratio of $4:1 \text{ to } 10:1 \text{ Fe:As/Se}$. Amorphous ferric hydroxide precipitates scavenge oxyanions via surface complexation:

Fe^{3+} + 3H_2O \longrightarrow Fe(OH)_3(s) + 3H^+

\equiv Fe-OH + HAsO_4^{2-} \longrightarrow \equiv Fe-HAsO_4^- + OH^-

3. Mechanical & Process Design Standards

Design of FGD ZLD equipment must strictly comply with international pressure vessel, heat exchanger, and piping codes due to high thermal stresses, corrosive electrolytes, and high-pressure operations.

Applicable Standards & Codes

Metallurgical Selection & Corrosion Metrics

In FGD evaporation systems, chloride concentration rises from $15,000 \text{ mg/L}$ in feed up to $150,000 - 200,000 \text{ mg/L}$ in the final crystallizer, while operating temperatures range between $55^\circ\text{C}$ and $115^\circ\text{C}$. Alloys must be selected based on their Pitting Resistance Equivalent Number (PREN):

PREN = \% Cr + 3.3(\% Mo + 0.5\% W) + 16(\% N)

Chloride Concentration vs. Alloy Selection Matrix
========================================================================================
Chloride Level (mg/L)   Operating Temp (°C)   Recommended Alloy          PREN Index
----------------------------------------------------------------------------------------
< 3,000                 < 60                  Stainless Steel 304L       ~ 19
3,000 - 15,000          < 80                  Stainless Steel 316L       ~ 25
15,000 - 45,000         < 95                  Duplex 2205 (UNS S31803)   ~ 35
45,000 - 90,000         < 105                 Super Duplex 2507          ~ 43
90,000 - 150,000        < 115                 Alloy 31 / Nickel C-276    ~ 47 - 68
> 150,000 / Slurry      > 115                 Titanium Grade 2 / 12      > 75
========================================================================================

[!CRITICAL] Crevice Corrosion & SCC Threshold: Standard austenitic stainless steels (304L, 316L) suffer immediate stress corrosion cracking (SCC) when chloride levels exceed $10,000 \text{ mg/L}$ at temperatures above $60^\circ\text{C}$. For high-chloride Multi-Effect Evaporator (MEE) and Forced Circulation Evaporator units handling FGD brine, Hastelloy C-276 (UNS N10276) or Titanium Grade 2 must be specified for heat exchanger tubes and vapor separation chambers.

4. Sizing Equations & Thermodynamic / Mass Balance Logic

1. Overall Mass & Solute Balance

For an N-stage evaporation train receiving feed mass flow \dot{m}_F with solute concentration C_F:

\dot{m}_F = \dot{m}_D + \dot{m}_B

\dot{m}_F  ·  C_F = \dot{m}_B  ·  C_B \implies Concentration Factor  (CF) = (C_B) / (C_F) = (\dot{m}_F) / (\dot{m)_B}

Where \dot{m}_D is total distillate vapor mass flow rate, and \dot{m}_B is concentrated blowdown brine stream flow rate.

2. Boiling Point Elevation (BPE) & Activity Coefficients

In high-TDS FGD brines containing mixed electrolytes (NaCl, CaCl_2, MgCl_2, CaSO_4), boiling point elevation (Δ T_{BPE}) reduces the available temperature driving force across evaporator effects. Using Bromley-Pitzer electrolyte thermodynamics:

Δ T_{BPE} = i  ·  K_b  ·  m  ·  \gamma_{\pm}

Where:

• i = van 't Hoff dissociation factor
• K_b = ebullioscopic constant of water ($0.512 ^\circ\text{C}\cdot\text{kg/mol}$)
• m = total molality of dissolved ionic species (mol/kg)
• \gamma_{\pm} = mean ionic activity coefficient of the multi-electrolyte mixture

For an FGD brine at $120,000 \text{ mg/L } Cl^-$ concentration (TDS ≈ 210,000 mg/L), measured Δ T_{BPE} ranges from $9.5^\circ\text{C}$ to $14.2^\circ\text{C}$.

    Effective Temperature Driving Force Profile across Evaporator Tube Wall
    
    Steam Temp (T_steam) ───────┐
                                │ ◄── Gross ΔT
                                ├── Hot Fluid Boundary Layer
                                │
    Boiling Temp (T_boil)  ─────┼─── Boiling Point Elevation (ΔT_BPE)
                                │
                                ├── Hydrostatic Head Loss (ΔT_hydro)
                                │
    Effective Vapor (T_vap) ────┴─── Net Driving Force (ΔT_net)

3. Evaporator Heat Transfer & Surface Sizing

Net temperature driving force (Δ T_{net, k}) for effect k is expressed as:

Δ T_{net, k} = T_{steam, k} - T_{boil, k} - Δ T_{BPE, k} - Δ T_{hydrostatic, k} - Δ T_{losses}

The total required heat exchanger surface area (A_k) for effect k transferring duty Q_k is calculated by:

Q_k = \dot{m}_{V, k}  ·  \lambda_{v, k} = U_k  ·  A_k  ·  Δ T_{net, k} \implies A_k = (\dot{m}_{V, k}  ·  \lambda_{v, k}) / (U_k  ·  Δ T_{net), k}

Where:

• \dot{m}_{V, k} = rate of vapor generated in effect k (kg/s)
• \lambda_{v, k} = latent heat of vaporization (kJ/kg)
• U_k = overall heat transfer coefficient (W/m²·K)

Typical overall heat transfer coefficients (U) for FGD brine units:

• Falling Film Evaporator (clean brine): U = 1,800 - 2,400 W/m²·K
• Forced Circulation Evaporator (high TDS/scaling): U = 1,200 - 1,700 W/m²·K
• Agitated Thin Film Dryer (pasty solid phase): U = 600 - 950 W/m²·K

4. Fluid Dynamics & Scale Suppression in Forced Circulation

To prevent calcium sulfate (CaSO_4) gypsum scaling on heat transfer surfaces during concentration above solubility limits, boiling inside tubes must be completely suppressed by maintaining hydrostatic overpressure and high fluid velocities:

v_{tube} = (4 \dot{V}) / (π D_i² N_{tubes)} \ge 2.2 - 2.8  m/s

Wall shear stress (\tau_w) required to prevent wall attachment of precipitating crystals:

\tau_w = (1) / (8) f  ·  ρ  ·  v_{tube}² \ge 18  Pa

Where f is Darcy friction factor and ρ is brine density (≈ 1,180 - 1,250 kg/m³).

5. Comparative Technology Selection Matrix

       Technology Selection Matrix for FGD Blowdown ZLD Plants
       =======================================================

      [ Option A ]                   [ Option B ]                  [ Option C ]
 Thermal MEE + ATFD             [MVR Evaporator](/process/equipment/mvr-evaporator) + MVR           High-Pressure RO + 
 (Forced Circulation)              Crystallizer                MEE + Spray Dryer

6. Real-World Case Example: 2 x 660 MW Coal-Fired Thermal Power Station

Plant Background & Design Criteria

A $2 \times 660 \text{ MW}$ supercritical coal-fired power station operating a wet limestone-gypsum scrubber was mandated by environmental authorities to achieve Zero Liquid Discharge. The raw FGD blowdown exhibited extreme scaling potential, fluctuating chloride levels, and heavy metal concentrations.

+-----------------------------------------------------------------------------------+
|                        DESIGN INLET BLOWDOWN WATER QUALITY                        |
+---------------------------------------------------+-------------------------------+
| Parameter                                         | Value                         |
+---------------------------------------------------+-------------------------------+
| Raw FGD Blowdown Flow Rate                        | 45.0 m³/h (1,080 m³/day)      |
| pH Range                                          | 5.2 - 5.8                     |
| Total Dissolved Solids (TDS)                      | 34,500 mg/L                   |
| Chloride (Cl⁻)                                    | 18,200 mg/L                   |
| Sulfate (SO₄²⁻)                                   | 7,400 mg/L                    |
| Calcium (Ca²⁺)                                    | 3,100 mg/L                    |
| Magnesium (Mg²⁺)                                  | 1,950 mg/L                    |
| Total Suspended Solids (TSS)                      | 1.5 wt% (Gypsum/Fly ash)      |
| Mercury (Hg²⁺)                                    | 2.4 mg/L                      |
| Arsenic (As)                                      | 0.85 mg/L                     |
| Selenium (Se)                                     | 3.10 mg/L                     |
+---------------------------------------------------+-------------------------------+

Installed ZLD System Architecture

 Raw Blowdown (45 m³/h)
         │
         ▼
 ┌────────────────────────────────────────┐
 │ 1. Primary Clarification & Softening   │ ◄── Lime, SDTC, FeCl₃, PAM Dosing
 └──────────────────┬─────────────────────┘
                    │ Precipitated Sludge to Filter Press (2.8 t/h wet cake)
                    ▼ Supernatant (43.2 m³/h)
 ┌────────────────────────────────────────┐
 │ 2. High-Pressure RO (HPRO) Unit        │ ──► Permeate Recycled (23.5 m³/h, TDS < 200 mg/L)
 └──────────────────┬─────────────────────┘
                    │ HPRO Brine (19.7 m³/h, TDS = 72,000 mg/L)
                    ▼
 ┌────────────────────────────────────────┐
 │ 3. 3-Effect Forced Circulation MEE     │ ──► Condensate Recycled (17.1 m³/h, TDS < 30 mg/L)
 └──────────────────┬─────────────────────┘
                    │ MEE Concentrated Slurry (2.6 m³/h, TDS = 310,000 mg/L)
                    ▼
 ┌────────────────────────────────────────┐
 │ 4. Agitated Thin Film Dryer (ATFD)     │ ──► Vapor Condensate Recycled (2.1 m³/h)
 └──────────────────┬─────────────────────┘
                    ▼
          Dry Solid Mixed Salt Cake (1.52 metric tons/hour, < 4% moisture)

Measured Performance & Operational Data

+-----------------------------------------------------------------------------------+
|                        SYSTEM PERFORMANCE & ENERGY METRICS                        |
+---------------------------------------------------+-------------------------------+
| Performance Metric                                | Achieved Operational Value    |
+---------------------------------------------------+-------------------------------+
| Total Water Recovery Efficiency                   | 95.6%                         |
| Overall Distillate Quality (Combined)             | TDS < 35 mg/L, Cl⁻ < 8 mg/L   |
| Final Solid Salt Moisture Content (ATFD Outlet)   | 3.4 wt%                       |
| Heavy Metal Removal in Pretreatment               | Hg > 99.4%, As > 98.8%        |
| Steam Consumption (3-Effect MEE + ATFD)           | 0.34 kg steam / kg water evap |
| Total System Power Demand                         | 18.2 kWh / m³ feed            |
| MEE Heat Exchanger Tube Metallurgy                | Hastelloy C-276 (UNS N10276)  |
| Operating Unplanned Downtime (Annual)             | < 1.5% (Zero Scaling Failures)|
+---------------------------------------------------+-------------------------------+

7. Engineering Best Practices & Operational Golden Rules

1. Gypsum Seeding & Saturation Control

Maintain a $1.0 - 3.0 \text{ wt%}$ gypsum crystal seed concentration in the recirculation loop of the first forced circulation effect. Precipitating calcium sulfate will preferentially crystallize onto circulating slurry seeds rather than forming tenacious scale on heat exchanger tube walls.

2. Shear Rate & Velocity Protection

Design forced circulation pump impellers for low shearing action (\le 1150 RPM) with pipe velocities maintained strictly between $2.2 \text{ m/s}$ and $2.8 \text{ m/s}$. Velocity below $2.0 \text{ m/s}$ permits salt settling and wall scaling, whereas velocity above $3.2 \text{ m/s}$ causes severe erosion-corrosion on titanium and Hastelloy surfaces.

3. Foaming & Organic Carryover Mitigation

Uncombusted coal organics and residual flocculants can cause severe foaming in evaporator flash chambers, leading to high TDS entrainment into condensate distillate. Install continuous anti-foam dosing systems (silicone-free polyether-based) and high-efficiency mesh-type demisters rated for \ge 99.9% droplet capture down to $5 \text{ }\mu\text{m}$.

4. Corrosion Prevention during Shutdowns

Never leave high-chloride brine standing stagnant inside heat exchangers during plant trips or turnarounds. Implement automated condensate flush cycles managed by PLC/DCS logic to completely displace brine from tubes with hot distillate within 15 minutes of any feed pump trip.

Related Equipment & System Solutions

• Multi-Effect Evaporator (MEE) – Thermal evaporation systems engineered for high-TDS industrial wastewater streams.
• Agitated Thin Film Dryer (ATFD) – Continuous thermal drying technology for converting sticky sludges into dry powders.
• Forced Circulation Evaporator – High-velocity evaporator designed to handle scaling liquids and high-solids slurries.
• Agitated Thin Film Evaporator (ATFE) – High-efficiency concentrated evaporation equipment for heat-sensitive and viscous applications.